Hypertension (HTN) and congestive heart failure (CHF) are the most important problems in contemporary cardiology. These chronic diseases account for most cardiovascular morbidity and mortality, and, despite much progress, remain therapeutic challenges. The cornerstone of therapy for both HTN and CHF includes the use primarily oral and intravenous drugs acting directly or indirectly on the kidney, such as angiotensin converting enzyme (ACE) inhibitors and diuretics, with the amount of each drug used dependent on the stage of the disease. While drug therapy is effective in the earliest stages of HTN and CHF, there is no truly effective drug treatment for the mid-to-later stages of these diseases.
HTN and CHF have many different initial causes. Irrespective of initial cause, both diseases follow a common pathway in their progression to end-stage disease, primarily as the result of excessive activity of the renal nerve. It has been shown in accepted animal models that renal denervation can control HTN and improve symptoms and slow down the progression of CHF. However, no drug or device therapies currently exist that can provide long-term, clinically usable blocking of renal nerve activity in humans. The only available clinical method of renal denervation is an invasive surgical procedure, technically difficult and of limited use, as the nerve quickly regenerates.
Of particular significance for this invention is the CHF condition that develops in many patients following a myocardial infarction (MI). Coronary artery disease causes approximately 70% of congestive heart failure. Acute MI due to obstruction of a coronary artery is a common initiating event that can lead ultimately to heart failure. This process by which this occurs is referred to as remodeling and is described in the text Heart Disease, 5th ed., E. Braunwald, Ch. 37 (1997). Remodeling after a myocardial infarction involves two distinct types of physical changes to the size, shape and thickness of the left ventricle. The first, known as infarct expansion, involves a localized thinning and stretching of the myocardium in the infarct zone. This myocardium can go through progressive phases of functional impairment, depending on the severity of the infarction. These phases reflect the underlying myocardial wall motion abnormality and include an initial dyssynchrony, followed by hypokinesis, akinesis, and finally, in cases that result in left ventricular aneurysm, dyskinesis. This dyskinesis has been described as “paradoxical” motion because the infarct zone bulges outward during systole while the rest of the left ventricle contracts inward. Consequently, end-systolic volume in dyskinetic hearts increases relative to nondyskinetic hearts.
The second physical characteristic of a remodeling left ventricle is the attempted compensation of noninfarcted region of myocardium for the infarcted region by becoming hyperkinetic and expanding acutely, causing the left ventricle to assume a more spherical shape. This helps to preserve stroke volume after an infarction. These changes increase wall stress in the myocardium of the left ventricle. It is thought that wall tension is one of the most important parameters that stimulate left ventricular remodeling. In response to increased wall tension or stress, further ventricular dilatation ensues. Thus, a vicious cycle can result, in which dilatation leads to further dilatation and greater functional impairment. On a cellular level, unfavorable adaptations occur as well. This further compounds the functional deterioration.
Takashi Nozawa et al reported the effects of renal denervation in “Effects of long-term renal sympathetic denervation on heart failure after myocardial infarction in rats” published in Heart Vessels (2002) 16: 51–56 Springer-Verlag. In rats the bilateral renal nerves were surgically denervated (cut) (RD) two days before MI was induced by coronary artery legation. Four weeks later, left ventricular (LV) function and sodium excretion were determined. In MI rats, RD improved the reduced sodium excretion. MI RD rats revealed lower LV end-diastolic pressure and greater maximum dP/dt as compared with those of MI innervation (INN) rats. LV end-diastolic and end-systolic dimensions were significantly smaller and LV fractional shortening was greater in MI RD rats than in MI INN rats.
Inventors described novel methods and devices for reversible minimally invasive modulation of the renal nerve in copending applications. This application describes novel drug delivery methods and integrated physiological drug delivery and sensing systems that provide a significantly more effective method of blocking the renal nerve for the purpose of treating HTN and CHF than are currently available. The objective of this invention is a fully implantable device that blocks renal nerve activity of at least one kidney that 1) can be placed in a minimally invasive manner and 2) requires minimal intervention by the patient and physician; and will greatly increase patient compliance leading to a higher overall effectiveness of these therapies. In addition, to HTN and CHF, this method may be applicable to other major diseases such as slowing the progression of chronic renal failure and reducing the number of patients requiring chronic hemodialysis.
Nerve blocking in humans is known and practiced mostly in the field of local anesthesia and pain control. While compounds utilized as general anesthetics reduce pain by producing a loss of consciousness, local anesthetics act via a loss of sensation in the localized area of administration in the body. The mechanism by which local anesthetics induce their effect, while not having been determined definitively, is generally thought to be based upon the ability to locally interfere with the initiation and transmission of a nerve impulse, e.g., interfering with the initiation and/or propagation of a depolarization wave in a localized area of nerve tissue. The actions of local anesthetics are general, and any tissue where nerve conduction, e.g., cell membrane depolarization occurs can be affected by these drugs. Thus, nervous tissue mediating both sensory and motor functions can be similarly affected by local anesthetics. Neurotoxins are the chemicals that when applied to nerve tissue in extremely small amounts can block a nerve for a period of time that significantly exceeds that achieved with local anesthetics. They are also more toxic and potentially more dangerous to the patient than local anesthetics.
Different devices and formulations are known in the art for administration of local anesthetics. For example, local anesthetics can be delivered in solution or suspension by means of injection, infusion, infiltration, irrigation, topically and the like. Injection or infusion can be carried out acutely, or if prolonged local effects are desired, localized anesthetic agents can be administered continuously by means of a gravity drip or infusion pump. Thus, local anesthetics such as bupivacaine have been administered by continuous infusion, e.g., for prolonged epidural or intrathecal (spinal) administration. For prolonged control of pain fully implantable pumps have been proposed and implemented. These pumps can store a certain amount of drug and a physician periodically refills those. Several authors proposed drug eluting implants for control of pain and muscle spasms that slowly release an anesthetic agent at the site of implantation.
The duration of action of a local anesthetic is proportional to the time during which it is in actual contact with the nervous tissues. Consequently, procedures or formulations that maintain localization of the drug at the nerve greatly prolong anesthesia. Local anesthetics are potentially toxic, both locally and via systemic absorption, yet must be present long enough to allow sufficient time for the localized pain to subside. Therefore, it is of great importance that factors such as the choice of drug, concentration of drug, and rate and site of administration of drug be taken into consideration when contemplating their use for the application to block renal nerve. Charles Berde in “Mechanisms of Local Anesthetics” (Anesthesia, 5th addition, R. D. Miller, editor, Churchill-Livingstone, Philadelphia 2000, pp. 491–521) stipulated that only 1–2% of the total amount of local anesthetic, when delivered by traditional methods, ever reaches the nerve. The rest of the drug is dissipated by circulation of blood that takes the drug away, not towards the nerve. It is therefore the purpose of this invention to maximize the amount of drug directed towards the nerve so as to achieve the effective blockade of the renal nerve with the minimal amount of drug.
Theoretically, a suitable commercially available implantable drug pump such as a Syncromed pump made by Medtronic Inc. (Shoreview, Minn.) can be used to block the renal nerve in a human. The pump can deliver common commercially available solution of a local anesthetic agent such as bupivacaine to the tissue surrounding the renal nerve via an attached catheter. Although feasible, such embodiment of the renal nerve block will have practical limitations. To block a peripheral nerve (for example, for the purpose of a commonly performed brachial plexus block) using conventional techniques the physician typically infiltrates 10–50 ml of bupivacaine or similar anesthetic into the tissue surrounding the targeted nerve. This usually achieves adequate blocking of both sensory and motor signals for 2 to 6 hours. Commercially available bupivacaine marketed as Marcaine or Sensorcaine is available in concentrations of 0.25 to 0.1%. For peripheral (single nerve) blocks concentrations of 0.5 to 0.75% are typically used. There are several reasons why local anesthetics are so diluted. An amino-amide compound such as bupivacaine can be toxic both locally (it is an irritant) and systemically (it depresses the heart). It is generally perceived that a local anesthetic will not be effective below certain minimum concentration and will be toxic above certain maximum concentration.
Implantable drug pumps are commonly equipped with an internal drug storage reservoir of 30 to 50 ml. Bigger reservoirs are possible but impose severe limitations on the physical and clinical acceptability of the implant. If the continuous (24 hour a day 7 days a week) block of the patient's renal nerve is desired, and a conventional peripheral nerve blocking technique is used, the implanted pump reservoir will need to be refilled every day or even more frequently. This is possible but not practical, since refilling of the pump is associated with the skin puncture, causing pain and leading to the risk of local and systemic infection. Also, daily infusion of a large amount of drug can result in a serious risk to the patient's health, especially if the patient has a weak heart. Notably the same drug bupivacaine is effective in a much lower doze when delivered directly to the targeted nerve tissue in the patient's spine. For example, an effective intrathecal (spinal) pain block can be achieved with 2–5 ml of bupivacaine. This observation shows that more targeted delivery of the same drug to the nerve tissue can result in 10 times or more reduction of the amount of drug needed for nerve blocking.
It is therefore the purpose of this invention to provide novel methods and implantable devices that will effectively block renal nerve by targeting the delivery of the selected drug to the nerve, reducing dissipation of the drug into the surrounding tissue, reducing the amount of drug stored in the device and increasing the time interval between the refilling or replacement of the device. It is also the purpose of this invention to enable testing of the effectiveness of the renal nerve blockade and to perform the renal block automatically, intermittently and/or periodically in the clinical scenarios where the continuous block is not desired.